A wire electric discharge machine is a machine that performs machining of a workpiece by use of an arc discharge between a wire electrode and the workpiece. In general, power feed contacts (feeding points) are respectively provided on the upper and lower sides of the workpiece (the wire electrode), and an electric current is supplied to the wire electrode via the power feed contacts. Machining currents flow from the upper and lower sides of the wire electrode simultaneously, so that two parallel circuits exist. Furthermore, a power supply is generally composed of a sub power supply circuit (a sub discharge power supply) for inducing a low-current spark discharge and a main power supply circuit (a main discharge power supply) for supplying a high current to be used as the machining current after the spark discharge is generated. Under rough machining conditions, the sub power supply induces a spark discharge, and the main power supply supplies a high current. On the other hand, under finish machining conditions, typically only the sub power supply is used for the machining. In other words, under the finish machining conditions, the sub power supply serves as the machining power supply.
In such a wire electric discharge machine, if machining currents, which are supposed to flow in parallel from the upper and lower sides of the wire electrode in a normal operating state, fail to flow from the both sides, i.e., the machining current from either one side does not flow due to some cause, the machining may become unstable because a high current is supplied from the other side. To cope with this problem, the following technology has been developed conventionally.
FIG. 8 is a schematic diagram of main elements of a conventional wire electric discharge machine, for example, disclosed in Patent document 1. In the wire electric discharge machine shown in FIG. 8, a machining current flowing from the upper side and a machining current flowing from the lower side are controlled by current-limiting resistors thereby achieving stable machining.
As shown in FIG. 8, an upper power feed contact 114 and a lower power feed contact 115 are respectively provided above and below a workpiece 112. First and second machining pulse power supplies 116 and 117 are provided with respect to the power feed contacts 114 and 115, respectively. The first machining pulse power supply 116 includes two switching transistors 119 and 120, a direct-current (DC) power supply 123, current-limiting resistors 121 and 122, and a pulse generator 118. Similarly, the second machining pulse power supply 117 includes two switching transistors 125 and 126, a DC power supply 129, current-limiting resistors 127 and 128, and a pulse generator 124. A single DC power supply can be shared by the first and second machining pulse power supplies 116 and 117 instead of the separate DC power supplies 123 and 129. Furthermore, a single pulse generator can be shared by the first and second machining pulse power supplies 116 and 117 instead of the separate pulse generators 118 and 124.
In a normal machining state, a current flowing through the upper power feed contact 114 and a current flowing through the lower power feed contact 115 are almost the same in magnitude. On the other hand, in an abnormal machining state, for example, where a current fed from either one side is suddenly stopped due to the effect of vibrations of the wire electrode or the like, a current fed from the other side increases, whereby the wire electrode tends to be easily broken. However, even in the state like this, in the wire electric discharge machine disclosed in Patent document 1, because the machining current flowing through the upper power feed contact 114 (or the lower power feed contact 115) is controlled by the current-limiting resistors 121 and 122 (or the current-limiting resistors 127 and 128), it is possible to prevent a flow of a high current from only one side. Thus, it is possible to prevent the wire electrode from being broken.    Patent document 1: Japanese Patent Application Laid-open No. S61-236427